In recent years greater importance has been placed on the natural gas as a new energy source and prospecting and developing works have been underway in the Arctic region to seek new gas fields. Along these prospecting and developing works, demands for high-tensile high toughness large-diameter gas line pipes and fittings which can realize efficient and economical transportation of the gas to consuming cities have been increasing rapidly.
Steel pipes for use in gas pipe lines are required to have an excellent ductility as determined by DWTT which represents the property of preventing brittle fracture as well as an excellent charpy impact value in order to prevent a large scale ductile fracture of the pipe lines and the fittings.
Conventionally, steel sheets for the gas line pipes which satisfy the above severe material properties have been produced by the so-called "controlled rolling method" (hereinafter called "CR"), and Nb-containing steels (hereinafter called "Nb-steels") have been mainly used for the purpose.
The Nb-steel is one of the most commonly used steel grades and has very excellent properties, but on the other hand the steel has lack of the following characteristics:
(1) In order to utilize Nb effectively for precipitation hardening and refinement of grains, it is necessary to dissolve coarse precipitation of Nb (CN) contained in the steel slab fully into solid solution during the heating of the slab. PA1 (2) When the steel slab is heated in a temperature up to 1150.degree. C. where Nb(CN) begins to dissolve into solid solution, the amount of Nb in solid solution thus obtained varies considerably due to fluctuations of the heating temperature. When the amount of Nb in solid solution increases excessively, the austenite grains (heated .gamma. grains) formed during the heating are in a remarkably mixed grain so that the toughness deteriorates, and even when the rolling is done under the same condition as in case of the steel slab in which the austenite grains are not in a mixed grain, the strength increases excessively so that the material quality lacks in stability. PA1 (3) Nb is an element which strongly prevents recrystallization of the rolled austenite grains (rolled .gamma. grains) during the rolling so that below about 1050.degree. C. no satisfactory recrystallization proceeds. Therefore, non-recrystallization (elongated austenite grain) of the austenite grains takes place before the grains are converted into fine recrystallized rolled austenite grains during the rolling, so that such difficulties are confronted as that the reduction amount is not enough in the non-recrystallization temperature zone, and that when the rolling is finished at a high temperature range in the non-recrystallization zone, the rolled structure thus obtained is a coarse mixed grain structure and susceptible to occurrence of the Widmanstatten structure particularly in case when a final plate thickness is thick. PA1 (4) When the warm rolling is strengthened, the yield ratio ##EQU1## reaches as high as 95% so that the production of the steel pipe, such as UO process becomes difficult to form to pipe, and deterioration of the yield strength due to the Bauschinger effect is considerable and thus excessive yield strength is required for the steel sheet. PA1 (5) During the welding of the steel sheet, precipitated Nb(CN) is apt to resolidify and the hardness increases remarkably, and toughness of the weld metal and the welding heat-affected zone (herein called "HAZ") deteriorates considerably. Also when the stress-relieve annealing (SR) is performed, the Nb which resolidifies during the welding precipitates to lower the toughness remarkably. PA1 (6) When a continuous casting process (CC) is applied for the slab production, Nb(CN) precipitates at the grain boundaries of the austenite grains to cause the intergranular embrittlement which leads to surface crackings of the steel slab.
However, precipitated Nb (CN) is stable at high temperatures so that it is not fully dissolved into solid solution at 1150.degree. C. or lower, and it is necessary to maintain a considerably long period of holding time for the heating, thus causing lowering in the productivity of the heating furnace.
The present inventors have conducted extensive studies for many years for development of a steel composition which overcomes the above defects of the conventional Nb-steels and still has the advantageous precipitation hardening property and the refinement of grains achieved by the conventional Nb-steels, and have found that addition of a very small amount of Mo is most effective for the purpose. However, it has been found that some molybdenum containing steel compositions show remarkable embrittlement when it is subjected to a warm rolling under certain rolling conditions. Then the present inventors continued to make studies also on the rolling conditions which cause the above embrittlement, and finally succeeded in developing the present steel sheet suitable for gas line pipes having excellent DWTT property.